(1) Field of the Invention
The present invention relates to an optical waveguide element suitable for a second harmonic generation device of a quasi phase-matching type and a process for producing the same.
(2) Related Art Statement
In the general optical information processing technology, the blue light laser that stably generates blue light around in a wavelength range of 400 to 430 nm at an output of not less than 30 mW has been demanded to realize high density optical recording, and competitions have occurred in developments. As the blue light source, an optical waveguide type frequency conversion device in which a laser generating an infrared as fundamental waves is combined with a quasi phase-matching type second harmonic generation element has been expected.
In the second harmonic generation device using a single crystal of lithium niobate, there is a limitation upon increase in the output of the light, because increase in the output of the light propagating the crystal changes the refractive index in the crystal through optical damage. The shorter the wavelength of the light, the more conspicuous is the optical damage. It is known that when a substrate of lithium niobate in which MgO is incorporated is used, it exhibits more resistance against the optical damage. At that time, the addition amount of MgO is around 5 mol %.
According to xe2x80x9cElectronics Letters, 24th April, 1997, Vol. 33, No. 9, pp 806 to 807, a optical waveguide type second harmonic generation device is realized by forming a periodically polarization-inverted structure at a substrate of MgO-doped lithium niobate, and forming a proton-exchanged optical waveguide in a direction orthogonal to this structure.
However, if the generated output of the blue light increases in the second harmonic generation device of this type, a stable output could not be obtained due to the optical damage. For example, when a periodically polarization-inverting structure is formed at lithium niobate doped with 5 mol % of MgO, a proton-exchanged optical waveguide is formed in a direction orthogonal to this structure, and a blue light having a wavelength of 420 nm is generated, the emitted beam and output largely vary due to the optical damage when the output is not less than 10 mW, particularly not less than around 15 mW. A cause for such variations in the emitted beam and the output was unclear.
It is an object of the present invention to reduce variations in output and realize stable resonance in an optical waveguide type device, even if the output of the emitted beam form the optical waveguide is increased.
It is another object of the present invention, in a frequency conversion element using a quasi phase-matching system, to shorten the wavelength of emitted light, preferably emit light in a blue range, and effect stable resonance with less variation even if the output in the emitted light through the optical waveguide is increased.
A first aspect of the present invention is to provide an optical waveguide element comprising a three-dimensional optical waveguide of a bulky non-linear optical crystal, a substrate, and a joining layer made of an amorphous material through which the substrate is joined to the optical waveguide.
A second aspect of the present invention is to provide an optical waveguide element comprising a three-dimensional optical waveguide of a bulky non-linear optical crystal, and an underclad for the optical waveguide, wherein the three-dimensional waveguide is formed in such a thickness by mechanically working the non-linear optical crystal as permitting light to be confined, and the underclad comprises an amorphous material.
A third aspect of the present invention is to provide a process for producing an optical waveguide element, comprising the steps of joining an optical waveguide-forming material of a bulky non-linear optical crystal to a separate substrate via a joining layer of an amorphous material, and forming a three-dimensional optical waveguide by working the optical waveguide-forming material, wherein the joining layer has a refractive index smaller than that of the non-linear optical crystal.
A fourth aspect of the present invention is to provide a process for producing an optical waveguide element, comprising the steps of joining an optical waveguide-forming material of a bulky non-linear optical crystal to a separate substrate via a joining layer of an amorphous material, and forming a three-dimensional optical waveguide by working the optical waveguide-forming material, wherein the joining layer has a refractive index smaller than that of the non-linear optical crystal.
A fourth aspect of the present invention is to provide an optical wave-length conversion element comprising a three-dimensional optical waveguide of a slab-shaped non-linear optical crystal, and clad layers of an amorphous material on upper and lower surfaces of the optical waveguide.
The present inventors discovered that when the three dimensional optical waveguide of the bulky non-linear optical crystal is joined to the substrate via the joining layer or directly to the substrate with no joining layer, and the joining layer or the substrate which is a base for the optical waveguide is used as an underclad, the optical damage is conspicuously suppressed and variations in the output of the light is prevented, if the light propagating in the optical waveguide is increased and/or the wavelength is shortened. As a result, the inventors reached the present invention.